Variable inductance tuner



Oct. 19 1948. 5, vQ 2,451,643 VARIABLE INDUCTANCE TUNER Filed Dec. 8, 1942 v 4 Sheets-Sheet l Fig - INVENTOR: sldney K Wh/fe AT TORNEYS S. Y. WHITE VARIABLE INDUCT'ANCE TUNER Oct. 19 1948.

Filed Dec.

4 Sheets-Sheet 2 9 INVENTOR. Sidney X Wh/fe AT TORNE Y5 Oct. 19 1948. 5, WHITE. 2,451,643

VARIABLE INDUCTANCE TUNER Filed Dec. 8, 19 42 v 4 Sheets-Sheet 4 F/q. l7.

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INVENTOR. S/dneyX W/I/Ie AT TORNE Y5 I Patented Oct. 19, 1948 l UNITED STATES PATENT OFFICE VARIABLE INDUCTANCE TUNER Sidney Y. White, Wilmette, 111., assignor to Victor S. Johnson, Chicago, Ill.; Alex Thomson administrator of said Victor S. Johnson, deceased Application December 8, 1942, Serial No. 468,195

7 Claims. 1

This invention relates to radio apparatus, and is particularly, although not exclusively, concerned with mobile radio apparatus suitable for meeting the exacting requirements of military service on land, at sea, and in the air,

Such apparatus may be carried on a wide variety of vehicles such, for example, as a truck, a tank, or a naval airplane catapulted from a cruiser. It may be subjected, therefore, to violent shock and to extreme and protracted vibration. Again it may be carried on the back of a soldier.

Such apparatus may be used in every condition of climate and weather to be found on the face of the earth, and in every season. It may be exposed to dust, mist, rain, sleet and snow from the air, and to mud and oil spray thrown up from roadways. It may be exposed to radical and abrupt changes of temperature, humidity and air pressure by being carried in the space of only a few minutes from a tropical desert into the substratosphere, or vice versa.

It is often desirable that many communication channels be made available, and that the distance at which a transmitter may be received shall be limited, in order that the same communication channels may be used in different regions without the possibility of mutual interference, and also as a safeguard against listenin in by the enemy. For these reasons, among others, it is often desirable that apparatus for military use be designed to operate in the ultra-high frequency range. The ultra-high frequency range has important uses for non-military as Well as military purposes, but its use imposes severe requirements.

The solution of the above problems is dealt with comprehensively and in detail in the present application and in divisions thereof, features originally shown but not now claimed herein being disclosed and claimed in the following divisional applications:

Radio apparatus, Ser. No. 506,372, filed October 15, 1943, Patent #2,407,359, dated September 10, 1946;

Radio apparatus, Ser. No. 725,685, filed January Electrical condensers, Ser. No. 506,374, filed Octo- Precision radio apparatus, Ser. No. 711,438, filed November 21, 1946.

A complete disclosure of illustrative apparatus embodying the invention may be found in Patent No. 2,407,359.

The present application has to do particularly with a variable inductance tuner per se, an important object being to provide a straight line tuning curve through a very extensive range.

A further object is to provide a unit of the kind referred to in the preceding paragraph in combination with means for materially altering the slope of the tuning curve without changing the straight line character thereof.

It is a still further object to provide a core operator and carrier which can be depended upon to determine and maintain a predetermined relation of the coil to the core and dial.

Fig. 1 is a fragmentary view showing the thrust rod and tuning core assembly;

Fig. 2 is a detailed sectional view showin a fragment of the structure shown at the left-hand end of Fig. 4 together with fragmentary portions of associated parts,

Fig. 3 is a top plan view of a coil and condenser supporting plate employed in the transmitter and in the receiver;

Fig. 4 is a sectional view of the supporting block shown in Fig. 3, taken on the line 44 of Fig. 3 looking in the direction of the arrows;

Fig. 5 is a sectional View of the block shown in Figs. 3 and 4 taken upon the line 55 of Fig. 4, looking in the direction of the arrows;

Fig. 6 is an end View of the assembly shown in Fig. 14.

Fig. 7 is a rear end view of a modified form of coil and condenser assembly;

Fig. 8 is a view in side elevation of the assembly of Fig 7;

Fig. 9 is a sectional View taken upon the line 9-4-1 of Fig. 7 looking in the direction of the arrows;

10 is a rear end View of a further modified form of coil and condenser assembly;

Fig. 11 is a View in side elevation of the assembly of 10;

Fig. 12 is a sectional view taken upon the line 12-42 of Fig. 10;

Fig. 13 is a sectional view taken upon the line lit-53 of Fig. 12 looking in the direction of the arrows;

Fig. 14 is a sectional View similar to Fig. 12 showing a further modified form of coil;

Fig. 15 is a graph illustrating the various relationships of tuning core movement in mils to the frequency in megacycles of circuits tuned and controlled by various diameter cores;

Fig. 16 is a graph showing mils of core movement required to change the frequency one megacycle in the range of 100 to 140 megacycles, for various coil and core combinations; and

Fig. 1'7 is a graph provided for use in explaining the alignment of the receiver and/or transmitter with the precalibrated dial.

Cores i2 and 12a are mounted upon a ceramic thrust rod iii for cooperating with the respective windings 35, 23 and Iii in fixed positions longitudinally of the rod for controlling the tunin of the tuned circuit assemblies containing .the windings in accordance with the longitudinal movement of the thrust rod I I I.

The dial mechanism, which will be described at a subsequent point, is designed to operate the cores I2 and lie. longitudinally for efiecting tuning. Operation of the thrust rod III from the dial mechanism is so contrived that no air lealn age joints are formed in the housing 86, 32.

The thrust rod III is provided with an enlargement groove I29 formed in it, as viewed in Figs. 1 and 2.. A hollow threaded cap I2I i impaled upon the. enlargement IIIa, and a resilient split ring I22 is caught Within the groove lie and captured within the cap I2I. With. the ring i22 thus captured Within the cap I2I, the cap 525 is threaded onto the cup H9 to force and maintain the lefthand end of the thrust rod III firmly against the base of the cap H9, so that the rods III and H are caused to move in unison as one unitary structure. The construction also serves to establish and maintain axial alignment of the rods III and H5.

At the rear or right-hand end of the rod Iii, the rod has affixed to it a metallic cap i22a having a circumferential groove !23 which is adapted to be received between inturned fingers of the yoke 89 of the diaphragm Bl. The cap serves as a slide bearing for cooperating with the bushing H3. A metallic sleeve I23d fixed on 1e rod I II serves as a slide bearing for cooperating with bushing I I2.

The mounting of the cores i2 and Ma upon the rod III (Fig. 1) presents adifilcult problem. A ferromagnetic core made up of small dust particles held together by a binder is decidedly fragile and readily tends to chip and crack, if any pronounced localized pressure is brought to bear on it. Anything in. the nature of a forced fit on the rod is impracticable.

Cores may be molded on the rod, but great care must be employed that the many tons of pressure required to form the core toa homogeneous mass does not crack the ceramic rod.

Cementingthe core on the rod offers the difficulty of lack of the extremely precise maintained position of the core because of cold flow and bad thermal expansion characteristics of the cement, and also its change with age. Cements have also marked erratic changes of dielectric constant with temperature, and since they would be fully in the field of the coil, if used to cement the core in place, would give rise to an undesired change in frequency with temperature. The mounting mean shown in Fig. l overcomes these difiiculties by holding the cylindrical core firmiy between two shoulders which bear over the entire ends of the core, thus giving rise to no high unit pressures. The ceramic piece Ii'Ia establishes the distance of the core I2a from the end of the push rod H5, and while it may be made integral with the rod III, due to manufacturing difiiculties it may well be made as a separate piece cemented on as shown. Its ends are ground parallel to one another and normal to the axis, and care i taken iIIa. which has a circumferential when cementing that these faces are maintained normal to the axis of the rod iii. Core IN is then loosely slipped on the rod and the ceramic spacer iiib slipped on next. The outside diameter of this spacer is made slightly greater than the diameter of the core, since experience has shown that if the core rubs against the inside of the coil form some iron particles are rubbed on" the core and permanently deposited on the inside of the coil form causing a change in frequency due to this wandering of the iron.

The next ceramic spacer Iiic is then slipped on to establish a desired position of the mixer core I2 on the rod. Another ceramic spacer Il Id is slipped over the rod subsequent to mixer core i2, and a metallic sleeve I235; is rather tightly fitted over the rod at that point to cooperate w h a sleeve bearing H2 screwed into the partition 95. Another ceramic spacer IIic then determines the position of the core I2 which tunes the antenna circuit, and a ceramic spacer III 1 determines the distance between the final core and the end cap 322a.

All of these cores and spacers are more or less loose fit on the rod 5 i I, and they are maintained in relation by thrust pressure developed by the spring washer i227), which forces the whole assembly against the stop provided by the piece iiia. The end cap 222a which cooperates with the bearing H3 inset in the partition 3-! i then forced on with some position.

The cement used to fasten on ceramic end cap Mia and metallic end cap I220, is out of the field of any coil, and if, due to cold flow or age this cement creeps slightly, say of the order of onethousandth of an inch or so, this will be taken up by the spring washer I221), and the cores will not be altered in their positions relative to the abutment shoulder of cap IIla. While all of the spacer sleeves are of predetermined lengths within reasonable tolerances, it will be observed that the most important core, namely, the core I2a, which cooperates with the oscillator coil, does not depend upon any spacer sleeve for its position, but abuts at all times against the shoulder of the cap IIIa. This point is stressed for the reason that variations which are tolerable with respect to the cores I2 would be highly objectionable with respect to the core I2a. The diameters of the spacers Mo to Hi have been made smaller than the cores solely to save weight which might" give rise to inertia effects when the apparatus is used in locations where marked vibration and shock are encountered.

While the rods III, the spacers III-a to III f, inclusive, the base plates I65 and the coil forms I'iB have been. described as of ceramic material, it is to be understood that this term is intended to comprehend other insulating materials which have been found suitable such as glass and quartz. Plastics in general or" either the thermoplastic or thermo-setting type have been found unsuitable for this type of construction, with the possible exception of Micalyx, due to their cold flow, their non-cyclic cubic change with temperature, and their often erratic changes of dielectric constant with temperature.

The rod I25 is thrust rearward by the dial mechanism, and in turn thrusts the rod I I I rearward with it against the predetermined, light, combined spring action of the bellows I I land the diaphragm 81.

It is to be noted that core tuning, as distinguished from variable condenser tuning,v may pressure and cemented in conveniently be effected through limited travel of a thrust rod, as distinguished from operation of a rotary shaft, and, therefore, is well adapted for operation through air-tight connections as described. The fact that tuning is effected through operation of a thrust rod is, therefore, a very important factor contributing to the elimination of adverse humidity and pressure variations.

Referring to Fig. '7, the coil supporting form I130, is shown as comprising a generally cylindrical shaped tube composed of the same ceramic material of which the plate I99 is formed. The coil may be heated when applied to the coil form, so that it may develop tension through shrinkage as it cools. The coil form is also longitudinally slotted as at N9, the slot being tapered to accommodate the tongues III so that the slot I16 and tongues I'll provide means for locating the coil form in a definite position on the supporting plate I69. A material which will glaze is applied to the portions of the coil form and plate I 66 which are to be brought into contact with each other and the members then baked to glaze the material which thereupon unites the supporting blocks and coil form into a unitary rigid stable assembly. The ceramic material is preferably of such a nature that its surface contains a large number of small particles which project beyond the general surface level and puncture the skin of the ribbon H5 in numerous places, thereby entirely preventing any slippage of the ribbon on the coil form. The result is that the coil is maintained tightly in engagement with the coil form at all times and does not change in shape due to any changes in temperature or humidity. In other words, the coil and coil form are, as it were; locked together throughout the full length of the coil and the size and shape of the coil remain at all times the same as those of the coil form. This arrangement obviates any noncyclic variation in distance between one turn of the coil and another and also any non-cyclic variations in the diameter of the coil so that once the coil is wound, its inductance thereafter is not subject to non-cyclic variations due to temperature or aging. The ribbon of the coil is desirably a semi-elastic material such as sterling silver. Such material combines with high conductivity a softness permitting ready penetration by the coil form crystals and an elasticity capable of maintaining the required. tension. Pure silver has been found unsuitable because it does not have the required elasticity.

Referring to Figs. 7 and 8, a powdered iron slug I5 is secured against the lower surface of the block I99 by means of a pair of screws I" which pass through the slots I99. The inner face I18 of the slug i5 is arcuate in shape so that it may be moved inwardly into engagement with the surface of the coil form H3. The manner of adjusting the position of the slug I5 for controlling the slope of the tuning curve of the oscillator will be hereinafter described.

Referring to Figs. 7 to 9, the coil supporting form iliad, formed of ceramic material is glazed to the supporting block 595. The coil form Il3a has a cylindrical exterior surface. In this case the oscillator winding 95 consists of three turns I94, I95 and I99,which are unequally spaced, the turns I96 and I95 which are first entered by the core I2a being spaced closer together than the turns I95 and I94. Each coil portion consists of a thin metallic ring I9? which may be of silver or other suitable metal, and whose inside diameter is somewhat less than the outside diameter of coil form II3a. The ring 191 on one side is cut transversely, so that its ends I99 and I99 are spaced slightly apart when the coil has been slightly expanded and slipped along the coil form into position, after which its inherent resilience causes the coil to grip the outer surface of the coil form firmly and secure the ring in a definite predetermined position on the coil form in a position normal to the axis thereof. The end I99 of turn I94 is connected to the end I98 of turn I95 by means of a metallic connector 209 which is soldered to the coil ends (see Fig. 9). A similar connector 20! connects the end I99 of turn I95 to the end I98 of turn I96, so that the current in passing through the winding flows in the same direction through all the turns. The coil terminals I80, I89 and I9I are provided the terminals I89 and I89 supporting the condenser 59 and the projecting tongue Il9 on the terminal I80 being soldered to the end I99 of turn I94. The tongue I19 formed on coil terminal 589 is soldered to the end I99 of turn I99, while the tongue I99 on terminal I9I is soldered to the end I98 of turn l95, the connection of tongue I99 to turn I95 providing the intermediate tap 53 (Fig. 1) on the oscillator coil 35. Coil terminals I89 and I89 are held to the plate I96 by headed screws I82, and terminal I9I is similarly held to plate I66 by screw I92. The manner in which this assembly produces SLF (straight line frequency) will be hereinafter described.

Referring to Figs. 10 to 13 another construction of Winding for producing SLF is shown in which the ceramic coil form I'll-ibis provided with two grooves 292 and 293 disposed normal to its axis, and the groove 293 being somewhat deeper than 292, as shown in Figs, 12 and 13. The winding has only two turns I94 and l99, each turn being formed of the metallic ribbon I15. The projecting tongue 5 I19 of coil terminal 589 is soldered to the end 99 of turn 194 and the tongue I19 of terminal I89 is soldered to the end I99 of coil turn I99. The two turns are connected together by means of a thin metallic connector 299 which is disposed in a longitudinally extending groove 295 formed in coil form I131), the groove 295 sloping inwardly in the direction of turn 96 as illustrated in Fig. 27. One end of connector 294 is soldered to the end I99 of turn I99 and the opposite end of connector 294 is soldered to the end i923 of turn i99 as shown in Fig. 13, so that the current passes through both turns of the coil in the same direction. The metallic ribbon is held under tension by the connector 294, which connector is prevented from rotating by the groove 295 in the coil form so that th inner surface of the ribbon is maintained at all times in firm engagement with the coil form. The center tap on the coil is provided by means of the tongue I99a on the center coil terminal I9I, this tongue being soldered to the middle of the connector 294 as indicated in Figs. 10 to 12. It is thus seen that the illustrated arrangement provides an oscillator coil in which the turn are of different diameter and have no pitch, and also provides a ready means for tapping the coil at its midpoint.

The coil arrangement shown in Fig. 16 is generally similar to that shown in Fig, 12 and the corresponding parts are indicated by similar reference numerals. In this case the connector member 29 5a does not bear against the sides of the groove 295 in the coil form I130. The ends of each coil turn are provided with a pair of projecting ears 296 which abut against the wall of the groove 205 as shown. In securing the turns of ribbon to the coil form the ears 2% at one end of theribbon are placed against the wall of groove 205 and the turn of ribbon wrapped around the coil form in the groove therein, whereupon the ribbon may be heated by an electric current from a suitable source to cause it to expand considerably in length whereupon the pair of ears 206 at the other end of the coil will be slipped into the groove 295 and the ribbon permitted to cool; The contraction of the ribbon will place it under tension .and cause its interior surface to be pressed into firm engagement with the bottom wall of the grocve'in which it is seated.

In all of the tuned circuit assemblies described, other than that of Figs. 7 to 9, a ribbon having a thickness of about three mils and a width of fifty to seventy mils may be advantageously em- .ployed. These dimensions are cited by way of example, however, and not as defining practical limits.

The type of tuning employed is of the core type, and While ferrous cores will be mainly discussed, the-conductive type core, as for instance silver or copper, may well be used in some applications.

The tuned circuit must, therefore, be designed with the requirements of core tuning in mind, It is basic, however, that before We can tune the circuit over a range, the circuit without such tuning means must in itself maintain a fixed frequency to a high order of accuracy. It must also allow trimming, tracking and aligning with a precalibrated dial having great length and accuracy of resetting. It must have no wiring at all.

The coil is first considered. Its design must The concentration of over 90% of the induct ance is actually in the coils of the tuned circuit assemblies described where it is capable of being acted on by a core.

The diameter of the coil i chosen to be about 405 mils in the present instance for use with a 375 mil core. Considerable difiiculty is had in the ceramic art in making thin walled tubes beyond a certain minimum thickness of wall, Maximum tuning ranges obtainable with core tuning are reached where the core substantially fills up the coil, but it must still freely pass through the bore of the coil form. If We chose this same ratio with a 125 mil core, the wall thickness would be less than mils, an impracticable figure for quantity production in the present state of the ceramic art.

The coil form is made with grooves for the conductor, to have the thick lands to support the thin grooves during firing, and also to guide in the winding.

Since the conductor chosen must have high conductivity, its thermal coefficient of expansion must also be high, at least two or three times that of the coil form. The wire must be wound under suificient tension and have enough elasticity to cling to the form at the most adverse temperature.

The cross-secticn of the conductor is a very thin strap, rather wide. If large, round conductors are used, such as #14 round wire, the current tends to hug the coil form as it is the smallest diameter of the turn. Any good conductor has a large temperature coefiicient of resistance,

however, and if the temperature be raised the resulting increased resistance causes a redistribution of the current, Causing the diameter of-the mean current path to be increased. This markedly increases the inductance, since diameter of the current path is square in the formula for the inductance coil, and great changes in frequency result.

By using a very thin strap of the order of three mils in thickness, this effect is minimized and a disciplined current path results. Instead of "using pure silver, sterling silver is used for greater toughness and elasticity and may be wound on the form quite hot b passing a heavy current through it while winding, in which case it shrinks on the form. Tension may be used also, sufficient to stress it nearly half way to its elastic limit so it hugs the coil form like arubber band.

Silver plated Invar or Nilvar used in large cross-section maintains its cross-section under temperature variation, but the current redistribution the same as for pure silver, and it must be wound under tension and in general has no advantage over the thin sterling silver strap, which may be flattened wire.

It is of great advantage to use ceramics of the low loss type such as Alsimag 196 because of the pre ence on the surface of minute sharp crystal structures which apparently pierce the skin of any unhardened metal pressed firmly against them. Repeatedtemperature cycling of these coils from 40 to +21. F. show no creepage of the win ing, since each unit length is captured by its adjacent crystals and held firmly in place.

The length of coil chosen must also depend in part upon the tunin curve desired and upon the length of core travel most easily obtained with a desirable dial mechanism. A coil 3'75 mils long, measured center of winding strap to center of winding strap gives an active core movement of about 250 mils for 25% tuning range.

In any coil to be used with a core the inside of the coil form must be left free to pass the corev Most methods of terminatingcoils use rivets, eyelets, or passing the conductor through holes in the form, all of which Would interfere wi h core movement. Some structure outside the simple cylindrical coil form is, therefore, required. This the form of the plate or block I65 with its associated terminal blocks m9, 889 and 59!,

The block 8% is preferably glazed to the coil form. Plastic cements are undesirable because of cold flow and change with age, but a good glaze in the joint fired at 1700 F. really makes the two pieces unitary.

There are numerous advantages in having a straight line relationship between the amount of displacement of a core which controls the frequency of a resonant circuit and the resonant frequency produced. For example, it may be desired to cause an increment in frequency of exactly one megacycle with a core movement of exactly 10 mils (3310"), and to have this relationship hold over as wide a tuning range as possible as, for example, from me. to me.

Among the advantages of such an arrangement are that the use of an essentially linear dial and movement is possible, and that it permits one resonant circuit to be readily tracked with another in cases where a definite relationship, such as a constant frequency difference, is to be maintained between the resonant frequencies of the two circuits. In tuning a circuit over this frequency range it has been found practically desirable, because of physical limitations, to arrange 9 for a total core movement of the order of ,4; inch, as giving the best balanced design in this instance.

Fig. 15 shows a generalized series of curves for the purpose of discussing design parameters in connection with the straight line frequency relationship. All curves except A have been displaced in frequency to avoid confusion.

If we consider the case where the coil is a two-- turn, flat ribbon spiral and the coil is essentially square, i. e., its diameter and length are about the same, and if this diameter be about .410 inch, then we may make up a series of cores of dilferent diameters and observe their eifect. A core the diameter of a pencil lead, for instance, would give us curve of Fig. 15, ans-shaped curve with no approximately straight section the middle. Curve D represents a somewhat larger core with greater tuning range, but still no substantially straight portion the center. A somewhat larger diameter gives us curve C having a small apparently straight portion, while further increase give us curve B with a substantially larger central straight portion,

Curve A represents the largest practical diameter that can be conveniently produced commercially without forcing us to thin the walls of the coil from beyond practical limits. This might have an approximately straight line frequency over a tuning range of about 17% in the frequency range shown. This shows the principle that the last few per-cent in tuning range increase that we obtain by steadily increasing the diameter of the core shows up as substantially straight line frequency (SLF). A spirally wound soil in general. does not give SLF. A coil stretched out over too much length is in general undesirable as the ends are too far apart physically, forcing the use of a long return path on the outside of the coil which gives us unwanted inductance in the tuned circuit, since only that portion of the wire in the circuit which is wound on the .coil form will be affected by-the core used, and any external inductance will lessen the tuning range and consequently the SLF range we may possibly reach.

The iron cores we have available at present that can be in these frequency ranges consist in general of two types, the oxide type, and the carbonyl type consisting of spheres of the order of 5 microns in diameter, and often having differentiated internal str cture. Either type, when insulated and compressed into a core by the use of a binding agent, has an apparent permeability of the order of 2 when used in circuits oi this ature, and we often wish to extend the SLF por on of the curve to the maximum possible to be reached with this iron.

Curve A represents an actual case of the use of such cores, and as seen it covers a range of about 17% which is substantially straight line frequency. It will be noted that the abscissa shows a total core movement of about inch or 500 mils, and that the SLF portion covers approximately zoo mils in the center of the curve. Drawn to this scale the curve seems to be absolutely straight in the center portion, but it is actually continuously curved by a rather slight but important amount.

There are a minimum of four effects taking place as ferrous core is introduced into such a coil for producing the curve A shown in Fig. 15. Thgy re: the true permeability effect alone; the eddy current effect which tends to cancel the permeability effect; the fact that the core is a capacity by itself and is also effective in grad- 10 ually capacity coupling one part of the circuit to another; and a resistance loss which is present in all apparatus. I'he curves shown, therefore, are the result of summation of these four effects. It has been found helpful to plot curve A again Fig. 16 in a different manner. A rather simple way to look at SLF is to say that to advance one unit of frequency we must advance the core one unit of distance, and as long as this relationship is maintained we have SLF tuning. The c es of Fig. 16 have, therefore, been plotted w In the ordinates representing the mils of core movement required to tune the circuit over one megacycle, and the abscissas representing a portion of the frequency spectrum we are investigating. Pl tted this way curve A is shown as A and forms a curve resembling a parabola. Curve A, Fig. 15, might be considered sufficiently SLF throughout the middle portion of its range to a degree of accuracy termed eye accuracy, that is, to a degree of accuracy that would appear to give equal graduations by simple examination. l-iowever, Fig. 16 shows that in absolute units it departs quite markedly from SLF, sumciently so that the use of a straight line dial mechanism is impracticable over the 17% tuning range shown.

Considerable investigation has been made of e use of tapered cores or cores with various profiles that could straighten out our SLF portion into a fairly high degree of accuracy over a tuning range of 8% to 10% as shown in curve E, Fig. 1E, but since curve A is drawn for the condition of the largest practical core in the form of a practically solid cylinder, it will be realized that we can only shape the core by removing material, and consequently our total tuning range is less, as can be seen by examination of F. Referring back to Fig. 15, it can be seen that the SLF rapidly disappears as we reduce the amount of metal in the core, so that this method of straightening up the SLF runs against a severe limitation if we wish to obtain the maximum possible effect.

Curve F was obtained with a 4% taper of a cylindrical core inch long with the small end entering the coil first.

Approximately the same result was secured by short sections of cylinders, cylindrical cores separated critical distances by insulating washers, and also by localized tapering of the core such as notching, and reducing diameters at various places by critical amounts. In general, no very pronounced phenomena were encountered, as the core seemed to work largely on the group field, and minor detail on the core did not matter.

t is the nature of a spiral coil of only two or three turns spread over a length equal to its own diameter that the turns have considerable pitch, and as a consequence the flux by no means along the axis, but is affected by the pitch of the end turn.

An entirely different line of attack is shown by the construction illustrated in Figs; 7 and 8, where a no pitch winding is used. Here each turn, except for a rather negligible cross-over connector, is exactly normal to the axis of the coil, and investigation showed the flux to be almost purely axial. By critical spacing of the three turns shown in Fig. 8, the rate of increase of the flux can be controlled, especially of the mutual inductance between the turns. When the turns of the coil of Fig. 8 are equally spaced by a distance of mils (edge of one turn to i th I1 edge of the next) a tuning curve similar to-dotted curve G is produced. It will be noted by comparing this curve with curve A that whereas the latter curve slopes continually downwardly toward its center portion, the effect of the mutual inductance between the turns has been to cause the center portion of the curve to be picked upwardly to a very substantial extent, as indicated.

It has been possible to utilize this variation caused by the mutual inductance between the turn to assist in the production of SLF to the 'extent shown in curve H by choosing th proper distance between the coil turns and ti e proper core diameter.

This curve H was produced with a small spacing between the two turns on the side of the coil which the core normally enters (61 mils), and an exaggerated. spacing to the next turn (125 mils). The diameter of the inside of the winding was 430- mils', and the core diameter was 365 mils to give the result shown, which is an SLF range of 22%. The core length was 500 mils, the hole through the core being 18? mils in diameter. The coil was wound with copper strap 71 mils wide and 35 mils thick.

It is believed to be possible to increase the tuning range by reducing the diameter of the coil form somewhat, but because of inability to pro cure special ceramic coil forms required it has not been possible for the present to investigate the problem further in this direction.

Considerable experience has shown that the straightness of curve H is adequate for use with the straight line tuning mechanism over a range of 22 megacycles, so that it is possible to use a dial of the type shown in which the dial graduations are made directly in terms of frequency.

Where it is desired to establish tuning ranges in the portion of the spectrum above 160 megaeycles or so, and still maintain a high order of circuit stability by having a good sized tuning condenser, a two-turn coil must be used. It has been found insufic'ien't merely to vary the distance between these two turns to obtain SLF, or any other desired curve, so a new variable is introduced, the choosing of difierent diameters of the two turns, with the turn on the entering side of the core the smaller. The coil in which turns are of different diameters is shown in Figs. to 14, inclusive.

The use of a ferromagnetic core has been assumed in the above description, as well as a ferromagnetic slug. It is known that in the case of a coil intended to be used at audio frequency with an ordinary laminated iron core, that the inductance of the coil itself may be increased a rather small amount by inserting only a laminated core as, for example, ten times. If the core be then removed and the entire outside magnetic circuit closed with a shell of laminated iron, the inductance will be raised also by a similar amount, as for example, ten times. Now if the core be reinserted so that the magnetic path is completely iron, the inductance value is increased by a very large amount, such as a total of one thousand times, as there is now no air gap in the magnetic path.

It has been found this holds true in ferromagnetic circuits at frequencies of the order of 100 to 300 megacycles, but since the ferromagnetic material is finely divided and contains numerous air gaps itself, the effect is much less pronounced. While the external slug may be a complete shell, and an extended tuning range may be obtained by such a shell in combination with a movable core, our purpose of changing the tuning slope, or rate of change of frequency for a given core displacement, is achieved by a rather small crosssection slug arranged to be brought close to, or withdrawn from, the side of the coil. This is mainly to allow for commercial tolerances in coils, condensers and tubes, and a slug of the size and disposition shown in Fig. 7 is adequate for our present purpose. While this feature of the invention has been described in connection with a coil of low inductance and few number of turns of a resonant circuit tunable over a range of ultra-high frequencies, it is equally suitable for adjusting the slope of circuits tunable within lower frequency ranges including the present broadcasting range and audio frequency ranges. The coils of such circuits may have a much larger number of turns, such as several hundred, but the effect of the outside slug is similar.

Where it is desired to precisely adjust the resonant frequency of an oscillator or other resonant circuit to a single predetermined frequency, the capacity and inductive elements of the circuit may be made to tune to this frequency as close as practical considerations permit, leaving the precise adjustment tothe correct frequency to the manipulation of the slug [5. It will, therefore, be seen that a single self-contained unit, but without the movable cor-e l2a, permits radio ap paratus to be built to resonate at a single assigned irequency in a very simple, compact and stable form.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. A tuning coil of solenoidal form for ultra high frequencies comprising a plurality of nopi ch turns for directing the flux along the coil axis, cross connecting means joining the ends of adjacent turns to one another to produce additive inductive effects, said connecting means extending substantially in the direction of the coil axis,

nd a core arranged to move axially inside said coil, the coil turns being critically spaced from one another and being of difierent critically related diameters in relation to the core characteristics, whereby the combined eirect of the changes of self-inductance of the turns, dependent upon their respective diameters, and the mutual inductance of the turns, dependent upon their diameters and spacing, is made responsive to the core movement to produce a straight line relationship of core displacement to frequency.

2. A tuning coil of solenoidal form for ultra high frequencies comprising a plurality of nopitch turns for directing the flux along the coil axis, cross connecting means joining the ends of adjacent turns to one another to produce additive inductive effects, said connecting means extending substantially in the direction of the coil axis, and a core arranged to move axially inside said coil, the coil turns being critically spaced from one another and being or different critically related diameters in relation to the core characteristics, whereb the combined efifect of the changes of self-inductance of the turns, depen ent upon their respective diameters, and the mutual inductance of the turns, dependent upon their diameters and spacing, is made responsive to the core movement to produce a straight line relationship of core displacement to frequency, a massive ferromagnetic slug not substantially shorter than the coil, means supporting the slug alongside the coil with capacity for adjustment transversely of the coil axis, and means for securing the slug in selected positions of adjust ment relative to the coil, whereby the slope of the tuning curve may be adjusted while maintaining the straight line character thereof.

3. An inductance varying core assembly, adapted to cooperate with an inductance coil, said core assembly comprising a hollow cylindrical core, a ceramic supporting rod extending through the core, a member unitary with the rod and constituting an enlargement of the rod at one end thereof, formed with plane end faces normal to the rod axis, one of said end faces constituting a datum plane and the other constituting a shoulder at a fixed predetermined distance from the datum plane, and parallel to it. for engaging the core and controlling the position of the core relative to said datum plane, the core being mounted freely on the rod, a spacer sleeve means mounted freely on the rod at the opposite end of the core from said shoulder, and resilient means affixed to the end of the rod remote from the datum plane for transmitting pressure through the spacer sleeve means to press and maintain the core in engagement with said shoulder.

4. In combination, a coil unit and a core assembly cooperative therewith said core assembly comprising a hollow cylindrical ore, a ceramic supporting rod extending through the core, a member unitary with the rod and constitutin an enlargement of the rod at one end thereof, formed with plane end faces normal to the rod axis, one of said end faces constituting a datum plane and the other constituting a shoulder at a fixed predetermined distance from the datum plane, and parallel to it, for engaging the core and controlling the position of the core relative to said datum plane, the core being mounted freely on the rod, spacer sleeve means mounted freely on the rod at the opposite end of the core from said shoulder, and resilient means aflixed to the end of the rod remote from the datumplane for transmitting pressure through the spacer sleeve means to press and maintain the core in engagement with said shoulder, said spacer sleeve means including a sleeve element immediately adjacent the core and within the coil unit of slightly greater diameter than the core, whereby rubbing of the core against the coil unit is prevented.

5. In combination, a hollow cylindrical core adapted to tune a resonant circuit, actuating means therefor including a metallic push rod, a ceramic supporting rod extending through the core, a ceramic spacer member unitary with the supporting rod and constituting an enlargement of the supporting rod at one end thereof, formed with plane end faces normal to the rod axis, one of said end faces constituting a datum plane and the other constituting a shoulder at a fixed predetermined distance from the datum plane and parallel to it for engaging the core and controlling the position of the core relative to said datum plane, means maintaining the core in intimate abutting relation to the shoulder, and means for maintaining the end'face of the spacer which constitutes the datum plane in intimate abutting relation to an end face of the actuating push rod.

6. In combination, a hollow cylindrical core adapted to tune a resonant circuit, actuating means therefor including a metallic push rod, a

ceramic supporting rod extending through the core, a ceramic spacer member unitary with the supporting rod and constituting an enlargement of the supporting rod at one end thereof, formed with plane end faces normal to the rod axis, one Of said end faces constituting a datum plane and the other constituting a shoulder at a fixed predetermined distance from the datum plane and parallel to it for engaging the core and controlling the position of the core relative to said datum plane, means maintaining the core in intimate abutting relation to the shoulder, and means rigidly attaching the spacer to the push rod for establishing and maintaining the spacer in coaxial relation to the push rod and for maintaining the end face of the spacer which constitutes the datum plane in intimate abutting relation to an end face of the push rod.

7. An inductance varying core assembly adapted to cooperate with a plurality of coaxial inductance coils, comprising, in combination, a first hollow cylindrical core, a ceramic supporting rod extending through the core, a ceramic member unitary with the rod and constituting an enlargement of the rod at one end thereof, formed with plane end faces normal to the rod axis, one of said end faces constituting a datum plane, and the other constituting a shoulder at a fixed, predetermined distance from the datum plane and parallel to it, for engaging the core and controlling the position of the core relative to said datum plane, the core being mounted freely on the rod, spaced sleeve means comprising a series of spacer sleeve elements mounted freely on the rod at the opposite end of said core from said shoulder, and resilient means to press and maintain the core in engagement with said shoulder, one of the spacer sleeve elements consisting of a second hollow cylindrical core, and another of the spacer sleeve elements constituting a metallic bearing sleeve located between the first and second cores.

SIDNEY Y. WHITE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,558,043 Priess Oct. 20, 1925 1,743,039 Kolster Jan, 7, 1930 1,887,470 Twort -1 Nov. 8, 1932 1,904,771 Hentschel Apr. 18, 1933 1,950,535 Young Mar. 13, 1934 2,014,650 Heintz Sept. 17, 1935 2,051,012 Schaper Aug. 11, 1936 2,059,299 Yolles Nov. 3, 1936 2,106,120 Lindberg Jan. 18, 1938 2,134,794 Muth et a] Nov. 1, 1938 2,137,392 Cobb Nov. 22, 1938 2,144,009 Barber Jan. 17, 1939 2,157,050 Bilger May 2, 1939 2,158,252 Polydorofi May 16, 1939 2,160,478 Laico May 30, 1939 2,177,835 Mennerich et a1. Oct. 31, 1939 2,186,184 Tubbs Jan. 9, 1940 2,246,239 Brand June 17, 1941 2,247,212 Trevor June 24, 1941 2,340,749 Harve Feb. 1, 1944 FOREIGN PATENTS Number Country Date 379,310 Great Britain Aug. 26, 1932 

